Plastic materials are used in numerous manufactured objects, because their injection, pressing, moulding, or calendering conversion methods make them easy to implement. These materials, made up of a resin, very often have a mineral or organic load added to them. One of the functions of said load is to substitute for the generally expensive resin, thereby making it possible to lower the material's overall cost.
These loads are numerous, such as talc, alumina, titanium dioxide, magnesium oxide, barium sulfate, aluminium, silica, kaolin, or natural or synthetic calcium carbonate, as indicated in the document JP 50-028105, which describes rigid materials obtained by mixing one or more of these loads with PVC.
These loads also make it possible to improve some of the final part's mechanical properties. For example, calcium carbonate makes it possible to improve the rigidity of PVC, its cooling time during extrusion, or its die build-up. This is particularly described in the document “The use of calcium carbonate to enhance the physical properties of rigid vinyl products” (Society of Plastic Engineering, Conf., 12-14 Oct. 1999).
Conventionally, the mineral or organic load is added to the plastic resin in the form of a dry powder. Said powder may result from a step of dry milling, or a step of wet-milling followed by drying, as this powder can even be treated before being added to the resin. Furthermore, numerous additives may also be added to this type of formula, such as impact modifiers, compatibilizing agents, thermal stabilizers, a lubricant, a rheology modifier, etc.
To that end, certain chemical additives with a very particular structure have for the past few years been the subject of increasing interest from formulators of loaded plastic compositions: they are (meth)acrylic comb copolymers. This expression refers to a copolymer made up of an essentially linear skeleton that is (meth)acrylic by nature, onto which are grafted at least two side segments made up of at least one “macromonomer”. The term “macromonomer” refers to a polymer or copolymer that has at least one terminal group with an unsaturated ethylenic function.
In this manner, the document WO 2007/052122 demonstrates that such copolymers, implemented in loaded thermoplastic or heat-setting formulas, make it possible to improve their impact resistance without altering their rigidity. In loaded formulas and chlorinated resin-based formulas, these copolymers behave like highly effective compatibilizing agents with respect to calcium carbonate: this is the subject of the document WO 2008/053296, which discloses that the dispersion state of the load is improved in the resin, which leads to improved optical and mechanical properties. Finally, the document WO 2008/107787 describes the rheology-modifying function of such copolymers, which advantageously make it possible to reduce the viscosity of calcium carbonate-loaded PVC resins.
According to the teachings of the three aforementioned documents, these comb copolymers are obtained by causing a monomer, which is (meth)acrylic acid, to react with a macromonomer whose formula (I) is:R-(A-O)m—(B—O)n—R′  (I)
In this formula, m and n are whole numbers, at least one of which is non-zero, A and B refer to alkyl groups with 2 to 4 carbon atoms, R refers to a polymerizable unsaturated function, and R′ represents hydrogen or an alkyl group with 1 to 40 carbon atoms.
The production of such macromonomers is well-known to the person skilled in the art, who may, for example, refer to the document U.S. Pat. No. 6,034,208. By the chain -(A-O)m—(B—O)n—, the Applicant is referring to both a statistical distribution as one in the form of blocks of A-O and B—O units.
However, the Applicant has been able to show that a particular class of these copolymers, entirely advantageously and surprisingly, do resolve a fundamental technical problem inherent in the methods for converting loaded plastic formulas. This problem is that of the thermal stability of said formulas, as these formulas are intended to undergo operations of conversion through injection, pressing, molding, or calendering, generally at high temperatures.